Occasionally a system as a whole is said to have "discretionary" or "purely discretionary" access control as a way of saying that the system lacks mandatory access control. On the other hand, systems can be said to implement both Mandatory Access Control and Discretionary Access Control simultaneously, where Discretionary Access Control refers to one category of access controls that Digital Subjects can transfer among each other, and Mandatory Access Control refers to a second category of Access Controls that imposes constraints upon the first.